The present invention relates to a balance control circuit for controlling the difference in volume between right and left channels in a stereo system.
A conventional circuit of this type is constructed as shown in FIG. 1. In FIG. 1, reference characters Li, Ri and Gi designate input terminals to which audio stereo signals from circuits such as bass and treble circuits in a preceding stage are applied. A neutral potential (DC voltage) is superposed on the stereo signals. The terminal Li is connected through a resistor R.sub.1 and a capacitor C.sub.1 to a first terminal of a variable resistor VR.sub.1, the second terminal of which is connected to a second terminal of a variable resistor VR.sub.2. The terminal Ri is connected through a resistor R.sub.2 and a capacitor C.sub.2 to a first terminal of the variable resistor VR.sub.2. The variable resistors VR.sub.1 and VR.sub.2 are adjusted to control the balance in volume levels between the right and left channels. Further, the first terminals of the variable resistors VR.sub.1 and VR.sub.2 are connected through capacitors C.sub.3 and C.sub.4 to respective buffer amplifiers formed by transistors Q.sub.1 and Q.sub.2. Each buffer amplifier can be used as an impedance conversion circuit, a voltage amplifying circuit, or an isolation circuit if it is disconnected from ground. Further in FIG. 1, R.sub.3 through R.sub.6 designate resistors for determining DC baises, and R.sub.7 through R.sub.10, resistors for determining gains.
In the buffer amplifiers, stereo signals are provided at the collectors of the transistors Q.sub.1 and Q.sub.2. These stereo signals are delivered through coupling capacitors C.sub.5 and C.sub.6 to output terminals Lo, Ro and Go and to the following stage. A resistor R.sub.11 and a capacitor C.sub.7 form a ripple-eliminating power source filter.
The capacitors C.sub.1 through C.sub.4 are coupling capacitors for preventing the variable resistors VR.sub.1 and VR.sub.2 from being damaged by DC currents, that is, they block the application of DC voltages to the resistors VR.sub.1 and VR.sub.2 from the adjacent stages.
The resistances of the resistors R.sub.1 and R.sub.2 and the variable resistors VR.sub.1 and VR.sub.2 are selected so that, when the sliding contacts of the variable resistors VR.sub.1 and VR.sub.2 are positioned at the first terminals of the latter as shown in FIG. 2, stereo signals applied through the input terminals Li and Ri are not significantly attenuated. When the sliding contacts are moved upwardly in FIG. 1, only the signal applied through the input terminal Li is attenuated, while the signal applied through the input terminal Ri is maintained unchanged. In contrast, when the sliding contacts are moved downwardly in FIG. 1, only the signal applied through the input terminal Ri is attenuated while the signal applied through the input terminal Li is maintained unchanged.
As is apparent from the above description, in the conventional balance control circuit, a number of coupling capacitors (C.sub.1 through C.sub.4) are needed to prevent the application of DC currents to the variable resistors VR.sub.1 and VR.sub.2. Therefore, the circuit is disadvantageous in that it is relatively high in manufacturing cost.